Breast cancer is the second most common cause of death from cancer in women in Europe and North America. Annually, more than 1.3 million women are diagnosed with breast cancer worldwide, and approximately half a million die from the disease (Heneghan et al., 2009, J Oncol doi:10.1155/2010/950201). The involvement of estrogen in mammary carcinogenesis has been known for more than 100 years (Clemons and Goss, 2001, N. Engl. J. Med., 344:276-85). It is prevailingly known that estrogen signaling is mediated by two major estrogen receptors (ER), ER-α and ER-β (Weihua et al., 2003, FEBS Lett., 546:17-24), which share a common structural architecture. ER-α is often used to refer to a 66 kD protein that functions as a transcription factor and regulates the transcription of estrogen-responsive genes. ER-α is comprised of 6 domains, A-F (Evans, 1988, Science, 240:889-95). The A/B region contains a ligand-independent transactivation domain (AF-1). Regions C and E are responsible for DNA and ligand binding, respectively. A ligand-inducible transcription activating function (AF-2) is present in the ligand-binding domain D/E/F (Berry et al., 1990, EMBO J., 9:2811-8). Recent research revealed the existence of a truncated form of ER-α with a molecular weight of 46 kDa, which lacks the first 173 aa (AF-1 domain) of ER-α and is designated as ER-α46 (Flouriot et al., 2000, EMBO J., 19:4688-700). The full-length ER-α is therefore recognized as ER-α66. ER-α46 functions to inhibit the transcriptional activity mediated by the AF-1 domain of ER-α66 (Flouriot et al., 2000, EMBO J., 19:4688-700) and to signal a membrane-initiated estrogen pathway (Li et al., 2003, Proc. Natl. Acad. Sci. U.S.A., 100:4807-12). Previously, we identified and cloned a 36-kD novel isoform of ER-α66, ER-α36 (Wang et al., 2005, Biochem. Biophys. Res. Commun. 336:1023-7). ER-α36 is transcribed from a promoter located in the first intron of the ER-α66 gene and lacks both transcriptional activation domains (AF-1 and AF-2), but retains the DNA binding, dimerization, and partial ligand-binding domains. Additionally, it possesses an extra, unique 27-aa domain to replace the last 138 aa of the ER-α66. ER-α36 is predominantly localized on the plasma membranes and mediates membrane-initiated estrogen signal pathway (Wang et al., 2006, Proc. Natl. Acad. Sci. U.S. A., 103:9063-8) such as activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (the MAPK/ERK) signaling pathway (Segars and Driggers, 2002, Trends Endocrinol. Metab., 13:349-54). ER-α36 expression was detected in both ER-α66-positive and-negative breast cancer tumors (Lee et al., 2008, Anticancer Res. 28:479-83, Zhang et al., 2010, Oncogene, October 11, doi:10.1038/onc.2010.458). High levels of ER-α36 expression are also associated with tamoxifen resistance; breast cancer patients with tumors highly expressing ER-α36 benefit less from tamoxifen treatment (Shi et al., 2009, J. Clin. Oncol., 27:3423-9).
The discovery of microRNAs (miRNAs), which are ˜22 nt long, has brought new concepts to breast cancer research. A line of studies have shown that deregulation of mRNAs causes tumorigenesis and metastasis (Medina and Slack, 2008 Cell Cycle 7:2485-92). Further, growing evidence shows that miRNAs may be ‘master’ regulators that regulate cancer cell proliferation through different mechanisms. For example, miR-21 has been demonstrated to be an onco-microRNA (oncomiR) in breast tumorigenesis. Up-regulation of this miRNA causes aggressive malignant growth of breast cancer via regulation of the anti-apoptotic factors Bcl-2, TPM1, tumor suppressor PTEN, and PDCD4 (Ng et al., 2009, J Oncol, doi: 10.1155/2009/305420). MiR-206 significantly down-regulated ER-α by binding to the 3′ UTR (Adams et al., 2007, Mol Endocrinol 21:1132-47, Kondo et al., 2008, Cancer Res 68:5004-8). MiR-17-5p regulates malignant growth of breast cancer by targeting a transcriptional factor, AIB1 (Hossain et al., 2006 Mol Cell Biol 26:8191-201), which is a coactivator for nuclear receptors, such as ER-α. Overexpression of miR-125a/b suppressed the activities of two important tyrosine kinase receptors, HER2 and HER3 (Scott 2007 J Biol Chem 282:1479-86), which are often deregulated in breast cancer. Recently, miR-10b has been shown to be associated with progression and metastasis in breast carcinoma (Ma et al., 2007 Nature 449:682-8). The same group also found that miR-31 is inversely correlated with metastasis in breast cancer (Valastyan et al., 2009, Cell 137:1032-46). Another interesting study shows that down-regulation of let-7 miRNAs was observed in breast tumor-initiation cells (BT-IC), and an increased level of let-7 was detected during BT-IC differentiation (Yu 2007, Cell 131:1109-23). Restoration of let-7 in BT-IC reduced proliferation and mammosphere formation in vitro, tumor formation, and metastasis in NOD/SCID mice (Yu 2007, Cell 131:1109-23). However, the detailed mechanisms underlying let-7 regulation in breast tumorigenesis are still unknown.
Previous studies have shown that let-7 sequences are highly conserved in vertebrates and invertebrates. Expression of let-7 miRNAs can be regulated temporally during cancer development (Yu 2007, Cell 131:1109-23) and embryonic development (Grosshans et al., 2005, Dev Cell 8:321-30). For example, expression of let-7 miRNAs increases during differentiation and in mature tissue, but is barely detectable in embryonic stage (Gunaratne 2009, Curr Stem Cell Res Ther 4:168-77). Let-7 is also considered as a tumor suppressor to inhibit malignant growth of cancer cells by targeting RAS (Johnson et al., 2005, Cell 120:635-47), HMGA2 (Lee and Dutta 2005, Genes Dev 21:1025-30, Mayr et al., 2007, Science 15:1576-9), and c-Myc (Kim et al., 2009, Genes Dev 23:1743-8). Reduced expression of let-7 miRNAs has been observed in colon cancer (Michael et al., 2003 Mol Cancer Res 1:882-91), lung cancer (Takamizawa et al., 2004 Cancer Res 64:3753-6), ovary cancer (Dahiya et al., 2008, PLoS One 3:e2436) and breast cancer (Yu 2007, Cell 131:1109-23).